Satellite navigation systems (SNS's) allow small electronic devices to determine their location (longitude, latitude, and altitude) in within a few meters using time signals transmitted along a line of sight by radio from satellites. One example of a satellite navigation system is the Global Positioning System (GPS). Several other SNS's have been or are being developed, such as the Glonass (Russia) and Galileo (Europe) satellite navigation systems.
SNS's have three parts: the space segment, the user segment, and the control segment. The space segment consists of a number of satellites, each in its own orbit above the Earth. The user segment consists of small receivers, which can be hand-carried or installed on vehicles, such as aircraft, ships, tanks, submarines, cars, and trucks.
The control segment consists of ground stations that make sure the satellites are working properly.
In operation, SNS satellites each broadcast a signal that contains the orbital parameters of the satellite and a means of determining the precise time the signal was transmitted. The position of the satellite is transmitted in a data message that is superimposed on a code that serves as a timing reference. The satellite uses an atomic clock to maintain synchronization of all the satellites in the constellation. An SNS receiver compares the time of broadcast encoded in the transmission with the time of reception measured by an internal clock, thereby measuring the time-of-flight to the satellite. Several such measurements can be made at the same time to different satellites, allowing a continual fix to be generated in real time.
Location detection is not always welcome, and there are situations in which it may be desired to deceive a satellite navigation receiver. U.S. Pat. No. 6,396,432 B2, entitled “Method and Apparatus for the Deception of Satellite Navigation” describes one such method.